[#4554] ClausePresenter: don't attempt to call #display_label on a non-existant field configuration

This fixes an error that appeared when a user specified a search field that
does not exist.  It is essentially the same as upstream fix projectblacklight/blacklight#3442,
so if that is accepted and we start using a Blacklight release that includes
it, we can remove our customized version.

Author: sandbergja

app/presenters/orangelight/clause_presenter.rb

@@ -8,5 +8,12 @@ def label
         super
       end
     end
+
+    # We will no longer need to override #field_label when/if we
+    # use a release of Blacklight that includes
+    # https://github.com/projectblacklight/blacklight/pull/3442
+    def field_label
+      field_config&.display_label('search')
+    end
   end
 end

spec/presenters/orangelight/clause_presenter_spec.rb

@@ -16,4 +16,17 @@
       expect(subject.label).to eq 'NOT some search string'
     end
   end
+  describe '#field_label' do
+    context 'when the field config does not exist' do
+      let(:field_config) { nil }
+
+      it 'returns nil' do
+        expect(subject.field_label).to be_nil
+      end
+
+      it 'does not raise an error' do
+        expect { subject.field_label }.not_to raise_error
+      end
+    end
+  end
 end